Field of the Invention
The invention relates to a semiconductor assembly with a solder material layer and a method for soldering the semiconductor assembly, in which a semiconductor body formed of silicon is soldered to a metal carrier plate through a sequence of metal layers which, as seen from the silicon toward the carrier plate, includes an aluminum layer and a diffusion barrier layer before soldering.
Such semiconductor bodies are installed in semiconductor components, especially in power semiconductor components, which are on the market in large amounts. The sequence of metal layers as a rule includes an aluminum layer which is disposed on a silicon semiconductor body. The aluminum layer adheres well to silicon and forms a perfect ohmic contact, especially with p-doped silicon. According to the prior art, disposed on the aluminum layer is a diffusion barrier layer formed mostly of titanium or chromium which acts as an adhesion promoter and rearside barrier between a nickel layer disposed on the diffusion barrier layer and the aluminum layer. In the prior art, a noble metal layer is applied to the nickel layer directly or on a thin titanium layer that follows and serves for adhesion improvement. The noble metal layer is mostly formed of silver, gold or palladium and serves as an oxidation protection for the nickel layer.
In the actual soldering process a solder disk is conventionally placed between the thus metallized rearside of the semiconductor body and the metal carrier plate which usually is formed of tin or flux. In the soldering process, the solder layer placed between the carrier plate and the silver layer of the semiconductor body then melts, whereby the silver layer is then dissolved and the nickel layer that follows begins to be dissolve from the solder material and the solder connection is produced.
However, that generally known soldering process has great disadvantages. On one hand, the different coefficients of thermal expansion of nickel and the silicon semiconductor body cause mechanical tension which lead to high wafer bending (bending &gt;1000 .mu.m) especially in the case of thin semiconductor bodies (thickness .ltoreq.250 .mu.m).
This leads to difficult handling of the wafer which leads to increased errors in cassette positioning and which leads to increased danger of breakage during treatment of the wafer. That problem was heretofore solved by trying to minimize the thickness of the nickel layer to such a degree that the soldering still exhibited a sufficient adhesive strength. Regardless of reduced thickness of the nickel layer (.apprxeq.1 .mu.m), disc deformations of between 700 and 2000 .mu.m still occur during production, which lead to the above-mentioned problems.